The general principle of off-shore seismic prospecting consists in using a seismic source to give rise to a disturbance in the off-shore environment (for example by releasing a volume of air or steam into the water, by varying the volume of an immersed body, by an implosion, by a spark, etc . . . ) and in using sensors constituted by hydrophones towed by a prospecting ship or by geophones placed on the sea bed, to record seismic data for extracting useful information concerning the geology of the subsoil.
Seismic data comprises useful information (e.g. a succession of seismic reflection echoes) buried in background noise which one seeks to eliminate. The background noise is generated firstly by natural phenomena such as swell and microseisms which are random and non-localized in character, and secondly by human activity, often called "industrial noise", and that is generally localized in character.
In this latter category, the most troublesome for a prospecting ship is noise generated by a seismic disturbance caused by a second ship that is prospecting the same site, since such noise requires the first and second ships to operate in "time-sharing mode".
Pages 1677-1680 of the publication Geophysics, Vol. 56, No. 10, Oct. 1990, Tulsa, USA, discloses a method of reducing industrial noise caused by exploitation of oil deposits. That method is intended to eliminate industrial noise having a spectrum very different from that of the source used for prospecting, so it is not appropriate for eliminating disturbances due to an undesirable signal whose spectrum is close to that of the source used for prospecting, and it is therefore inapplicable to the case of two prospecting ships using seismic sources that generate disturbances having similar spectra.
Since no satisfactory method is available for eliminating industrial noise of the kind created by another prospecting ship, the ships must prospect in turn. Naturally, the period of inactivity during which a prospecting ship may not make any seismic emission has a significant effect on the cost of a prospecting campaign, thus making the cost of prospecting prohibitive in very busy areas that are rich in deposits, such as the North Sea.